In the field of working cylinders without piston rods, various types of force takeaway have emerged in practice. There are known fluid-driven working cylinders without piston rods, which have force takeaways guided on the outside of the cylinder tube. These can be coupled mechanically or magnetically with the piston. The mechanical connection can occur by means of an encircling transfer element, particularly a band or cable (EP-0 177 880 B1), or by means of a rigid linkage member between the piston and the force takeaway. In the latter case the linkage member passes through a sealable slit in the cylinder tube (EP-0 190 760 Al, DE 3509891 A1).
The sealing of the cylinder ends, through which the band or cable (wire rope) passes, is problematic with the known band or cable cylinders. Usually these bands are steel bands with almost a rectangular cross section. It is understandable that the sealing at the cylinder ends is extremely difficult to achieve during rapid movement of the piston to which the bands are fastened. The sealing lips of the seals must adjust themselves to the band within a short period of time.
In addition there is the following problem, since in practice steel bands with a thickness of 0.2 mm are usually used as bands. These are so sensitive, that due to the effect of stress concentration, which in turn is attributable to scratches on the surface--possibly caused by handling with a tool--they are prone to tearing.
The following problem occurs in the case of using a cable, namely, that the surface of the cable is in itself uneven, and hence leaks are to be expected from this.
In DE-PS 952 308 the usage of a chain or toothed chain as a "band" has been proposed, but in a working cylinder where the band space, which is separated from the piston space, is pressurized when pressure fluid is introduced into the piston space in order to move the piston in one direction or another. The described working cylinder in the referenced document is an internally pressurized working cylinder, in which the band space is not separated from the piston space in the sense of pressure.
WO 88/01698 A1 shows a further internally pressurized working cylinder without a piston rod. Therein the translational movement of the piston is converted to a rotary movement of the reversing pulleys and thereafter outside of the cylinder this rotary movement is converted again to a translational movement of a force takeaway. With this cylinder losses are of course incurred. The band is guided in a band space as in the case of the DE-PS 952308, which has been pressurized with the same pressure as the one side of the piston space. A stored sealing element, which slides in the band space, connects within the band space both ends of the band, which is shaped as a toothed belt. Since the cylinder is an internally pressurized cylinder, the band space is therefore not separated from the piston space in the sense of pressure. This sealing element basically works like a small piston, which is being pushed in the same direction as the working piston as a result of pressurization of the band space. This sealing element therefore entails a force as a planned feature which pushes against the piston by reason of its effective surface, which is set against the active force of the piston, whereby the efficiency of the cylinder is drastically reduced.
In cases of a working cylinder with a stiff linkage member between the piston and force takeaway, the problem of leakage develops first at the entry point of the linkage member. In addition one has to consider that, in the case of pressurization of the cylinder tube in order to move the piston therein, the cylinder will have a tendency to expand. Of course, the slit for the linkage member offers the ideal location for an expansion of the cylinder tube, during the momement of operation of the piston. This means that during pressurization of the cylinder tube, the leakage of the cylinder is increased. As a solution for this problem it has already been suggested to use a cylinder tube with variable wall thickness, such that the thickness of the wall is greater on the side opposite the slit (EP-0290760 A1, FIG. 2, DE 3509891 A). By this means the thicker material should work against a bowing out of the tube.
According to another suggestion (EP-0068088 B1) the force takeaway is provided in such a way to grip the cylinder tube in a clamp-style, in order to counteract the expansion in the area of the longitudional slit.
If one considers now that known working cylinders are driven with an approximate air pressure of 6-10 bar, it becomes clear that expansions of even a few tenths of a millimeter can drastically reduce the efficiency of such a cylinder.
In view of the above background it is now the object of the present invention to create a working cylinder without a piston rod, wherein the stated sealing problems of other known working cylinders are not present.